In a cellular communication system, a service coverage area served by a base station is called a cell. In a multicellular or macrocellular environment, a larger cell, or macrocell, may be further divided into smaller cells, or microcells in order to increase capacity. An overlay of macrocells may be implemented over the same area that an underlay of microcells are implemented. Thus, a microcellular network includes a plurality of microcells which may be partially disposed within at least one macrocell. A mobile station may be located in an area that is serviced by both a macrocell and a microcell.
In some cellular communication systems it is sometimes desirable to have more than one layer of cells. A particular example is a layer of microcells (e.g. a cluster of small cells a few hundred meters in diameter or less) existing under an "Umbrella" of a much larger cell or cells, usually known as macrocells. Other examples might include (not exclusively) concentric cells where an area of coverage is divided into an inner region near to the base station site and an outer annular region further from the base site, with each region treated as separate cells. A further example is that of conventional cells versus "Supercells" where "Supercells" in GSM (Global System for Mobile Communications), terminology means cells which include coverage of communications ranges in excess of 35 Km from the base station, often used for rural or maritime coverage. Another term for this is "Extended Cells".
Having established a multi-layer cellular network, it is often necessary to direct certain mobile stations (or subscriber units) from one layer to another layer and to hold a mobile station in one layer or the other. It may further be desirable to accomplish this, even though the normal cell selection procedure would result in the mobile naturally selecting a cell layer other than the one desired by the revised system operation. A particular example is in microcell/macrocell networks, where it is desirable to direct fast moving vehicular subscribers to a wide area macrocell network and slow moving pedestrian subscribers to a small area microcell network. In such communications systems, there may be no mechanism in the network to do this as part of normal systems operation. A particular example is the GSM phase one networks, which did not anticipate microcellular operation and in which all mobile subscribers (fast moving and slow moving) select the strongest cell (which would usually be the nearest microcell).
EP-A-0589279 describes a microcellular communication system having microcells overlaid by macrocells.
Thus, it is desired to have a method to induce a mobile station to select the cell layer preferred for system operation and to maintain the mobile on that layer as required.